From: email@example.com (N. Thornton)
Subject: Re: Class B amplifiers: what are the large low-value resistors for?
Date: 14 Nov 2002 17:14:24 -0800
NNTP-Posting-Date: 15 Nov 2002 01:14:24 GMT
Paul Burridge wrote in message news:...
> On Thu, 14 Nov 2002 17:34:21 +1100, "Phil Allison"
> opined thusly:
> > ** They *are* necessary, with no emitter resistors the amp would
> >probably self destruct when it got hot by a process called "thermal
> >runaway" - kind of a vicious cycle that overheats the silicon.
> But I'm getting thermal runaway anyway even with 2 * one ohm resistors
> > Using 1 ohm resistors will reduce the available power - possibly
> >by a large factor if the amp has "VI limiting" built in - most do.
> This one hasn't: it's just a breadboard experiment. BTW, there's no
> reactive load, just a loudspeaker. Okay, I know even a loudspeaker is
> technically a reactive load, but we're only talking low audio
> frequencies here, so I guess it'd be purely resistive, yeah?
> What I'm outlining here is class B *audio* of a few watts. Sorry if I
> should have made that clearer.
> > You may parallel connect several 1 ohms to get 0.33 or 0.25
> >ohms - that should work fine.
> AFAICS, reducing the inter-emitter resistances would only exacerbate
> the thermal problem!
I'm thinking you're not using class B here :) More like AB, which is
somewhat different. Class B would be simpler, and can in fact be made
to work without any problematic level of crossover distortion.
If you're willing to use geranium output trs you can do a much simpler
cct that's totally stable and sounds nice. I always do that for
Use an LM324 opamp to drive 2 small 1 watt Ge output trs, no bias.
Just connect the bases together and hook them straight to the LM324
output. A few Rs for the opamp feedback, one output cap, and thats it!
It only works with geraniums though: silicons give terrible crossover
dist. And its runaway proof, despite the lack of any obvious runaway
Theres no thermal or short protection, and no capacitive load
proofing. Them is extra!